Java中用TreeSet方法排序字符型数字怎么排序

import java.util.*;
import java.lang.*;
import java.io.*;

class Demo implements Comparable{
    String str;

    public Demo(String str){
        this.str=str;
    }
    public void setStr(String str){
        this.str=str;
    }
    public String getStr(){
        return str;
    }
    //重写toString()方法
    public String toString(){
        return  " "+this.str;
    }
    //比较规则
    public int compareTo(Object arg0) {
        Demo d2 = (Demo)arg0;
        if (d2.getStr().length() != str.length()) return str.length() - d2.getStr().length();
        return str.compareTo(d2.getStr());
    }
}

class SortByNum implements Comparator{

    public int compare(Object arg0, Object arg1) {
        Demo d1=(Demo)arg0;
        Demo d2=(Demo)arg1;
        return d1.compareTo(d2);
    }
}

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        //创建一个TreeSet集合
        TreeSet tree=new TreeSet(new SortByNum());
        //添加字符
        String s = "8 10 15 2 5 7";
        for (String s1 : s.split(" "))
            tree.add(new Demo(s1));
        System.out.println(tree);
    }
}